1. Field of the Invention
The present invention relates to an amplifying device, and more specifically relates to the amplifying device for amplifying audio signals of a plurality of channels.
2. Description of the Related Art
Audio signals of a surround system have a plurality of channels. Some types of digital amplifiers can amplify audio signals of respective channels respectively. Such digital amplifiers have amplifying sections corresponding to channels, respectively, and each of the amplifying sections modulates an audio signal (analog signal) of corresponding channel into a digital signal so as to amplify it.
Since a number of surround system channels varies from 2.1 ch to 7.1 ch, the digital amplifiers do not amplify audio signals of all channels in some cases. For this reason, the digital amplifiers need to be controlled so that the amplifying sections corresponding to unused channels do not amplify audio signals.
Each of the amplifying sections of each channel uses MOSFET (Metal-Oxide-Semiconductor Field-Effect Transistor) in order to amplify a digital signal obtained by modulating an analog signal (audio signal). Driving ICs for driving the MOSFETs are provided by various vendors. Non-Patent Document discloses a driving IC “IRS2092” manufactured by International Rectifier “Application Note AN-1138 IRS2092(S) Functional Description”, Searched on Jul. 18, 2012, Internet <URL:http://www.irf.com/technical-info/appnotes/an-1138.pdf>.
Japanese Patent Application Laid-Open No. 2012-60269 describes an amplifier that can set channels to be amplified in a plurality of channels. In Japanese Patent Application Laid-Open No. 2012-60269, the amplifier includes a DSP (Digital Signal Processor), and a plurality of amplifying sections. The plurality of amplifying sections is related to channels, respectively, and includes a PWM (pulse width modulation) modulator and a switching section. The DSP inputs a digital signal and a carrier frequency into the amplifying section corresponding to a channel to be used, and inputs only the carrier frequency into the amplifying section related to an unused channel. In another manner, the DSP inputs signals for turning off the PWM modulator and the switching section into the amplifying section related to the unused channel.
In Japanese Patent Application Laid-Open No. 2012-60269, when the DSP inputs a carrier frequency into the amplifying section related to the unused channel, the switching section continues amplification in the amplifying section related to the unused channel. As a result, unnecessary radiation is caused, and power consumption increases.
The PWM modulator requires time until its operation is stable after the power is on. When the amplifying section related to the unused channel restarts amplification, the amplifying section cannot amplify an audio signal until the PWM modulator can stably operate.